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Floodplain
inundation is one of the major factors that has shaped
the geography, flora and fauna of California's Central
Valley. As in most developed regions of the western
United States, the majority of the historical floodplain
has been lost to dam and levee construction. However,
California was fortunate in that early flood engineers
retained relatively large areas of floodplain as part
of the flood management system. The largest contiguous
floodplain area of the lower Sacramento River (the primary
tributary of the San Francisco Estuary) is the Yolo Bypass,
a 24,000 ha basin. The Bypass is a central feature
of the Sacramento River Flood Control Project, which conveys
floodwaters from the major valley rivers including the
Sacramento, American, and Feather Rivers, and their tributary
watersheds.

Seasonal
inundation of the Yolo Bypass is one of the most dramatic
seasonal events in California's Sacramento Valley.
The Yolo Bypass typically floods in about 60 percent of
years, when high winter and spring floodwaters enter from
the Sacramento River and several small streams: Cache
and Putah creeks; Willow Slough; and the Knight�s Landing
Ridge Cut from the Colusa Basin. This has a major physical
effect on the San Francisco Estuary and its two component
regions, the Sacramento-San Joaquin Delta and downstream
Bays. Complete inundation of the floodplain approximately
doubles the wetted area of the Delta and is equivalent
to about one-third the area of San Francisco and San Pablo
bays. The floodplain is seasonally dewatered in summer
and fall, except for perennial ponds and a single tidal
channel. During extended droughts such as 1987-1992,
the floodplain is not inundated from its tributaries.

Although
flood control is the major function of Yolo Bypass, the
floodplain also supports agriculture, fisheries and wildlife.
The Yolo Bypass floodplain is dominated by agricultural
uses, but there are also substantial �natural� habitats
such as seasonal wetlands, riparian and upland habitat.
The largest contiguous area of non-agricultural floodplain
habitat is the Yolo Basin Wildlife Area, managed by California
Department of Fish and Game.